The present invention is directed to shaft arrangements in gas turbine engines. In particular, the present invention relates to an overhung mounting system for planetary gear trains.
In overhung mounting systems, a shaft within a gas turbine engine is supported by bearings at or near a first end, and a load is suspended, or cantilevered, at a second end of the shaft. Alternatives to overhung mounting systems include straddle mounting systems in which both ends of the shaft are supported by bearings and the load is positioned between the bearings. In each configuration, a pair of spaced apart tapered roller bearings can be used to provide thrust reaction and shaft stiffening. Straddle mounting systems provide stability to the shaft, but typically require greater shaft lengths to support the set of roller bearings in addition to a ball bearing. In gas turbine engines used as propulsion systems for aircraft it is desirable to reduce engine length and weight, particularly in engines utilizing epicyclic gear trains that use additional axial space.
Fan drive gear systems use epicyclic gear trains to reduce the output speed of a gas turbine engine in order to drive a large diameter fan. As the bypass ratio of the engine increases, it becomes advantageous to use higher gear reduction ratios. The epicyclic gear train is then configured as a planetary gear system, rather than as a star gear system, to provide higher gear reduction ratios. Planetary gear systems are supported axially between a gas turbine engine input shaft and an output fan shaft, and radially by a ground connection to a stationary engine component. In such a configuration, as with all epicyclic gear trains, it is desirable to maintain proper alignment of the shafts in order to reduce wear in the planetary gear system. Misalignment of the input and output shafts can result in wear of gear teeth.
Various aircraft maneuvers impact alignment of gas turbine engine shafts, which induces vibration of gear trains. For example, high gravity turns or hard landing operations induce flexure of engine cases, sometimes referred to as backbone bending, that is transmitted to the gear train as a bending moment that shocks the gear teeth. It is advantageous to permit the gear train to be radially displaced a limited amount to absorb flexure without damaging the gear teeth. Thus, overhung mounting systems are typically used with epicyclic gear trains because of their ability to incorporate radial displacement with the use of flexible shafts. Overhung mounting systems, however, permit too much flexure when support bearings are moved close to the gear train in an attempt to shorten engine length. There is, therefore, a need for an axially short and radially flexible shaft mounting system suitable for use with a planetary gear system.